1

2

Nonlinear Molecular Vibrational Spectroscopy for Surface Analysis of Thin

Complex Films and Buried Interfacial Systems (II)

NSC89-2112-M-009-036 (Aug. 1, 1999 to July 31, 2000)

Investigator: Jung Y. Huang

Organization: Institute of Electro-Optical Engineering

Chiao Tung University

Hsinchu, Taiwan 30050, R.O.C.

Tel: 03-5719755

Fax: 03-5716631

E-mail: jyhuang@cc.nctu.edu.tw

Abstract

In this report, we present the studies of thin complex films, such as liquid crystals

and polymeric aligning layers, with optical retardance measurement, infrared absorption

spectroscopy, Raman scattering, and Raman imaging microscopy. The molecular ordering

of liquid crystals and polymeric thin films induced by mechanical rubbing and ultraviolet

light exposure are investigated in view of their technical potential. An important question is

that how liquid crystal molecules are aligned and reoriented during field-induced switching.

It is still unclear about the detailed reorientation process of the chain and core parts of liquid

crystal. To understand the underlying dynamics at the molecular level, we are employing

Fourier transform infrared absorption spectroscopy to generate time-resolved spectra of

liquid crystal molecules during the field-induced switching process. By combining these

methods we wish in the near future to answer how molecular chirality and chiral dopants

affect the switching dynamics of ferroelectric liquid crystals.

3

Contents

I. Introduction

II. Theory

(a) Polarized infrared absorption and the relations to the order parameters of molecular

orientational distribution

(b) Raman scattering and the relations to the order parameters of molecular orientational

distribution

III. Experimental developments for characterizing thin complex films

(a) Characterization of polymeric aligning surfaces with optical retardation measurement

(b) Characterization of polymeric aligning films with Fourier transform infrared

absorption spectroscopy

(c) Anchoring energy measurement of liquid crystal molecules on a polymeric aligning

surface

(d) Raman imaging for thin complex films

(e) Time-resolved and polarized Fourier-transform infrared absorption spectroscopy for

liquid crystal alignment and field-induced reorientation

IV. Conclusions and future prospect

4

I. Introduction

Functional thin complex films such as polymers, liquid crystals, and ferroelectric

materials, possess high application potential. However, usefulness of these thin film systems

critically depends upon the spatial distributions of the constituents and their orientation

alignment. The information unfortunately is quite difficult to be obtained. We are

conducting a multi-years research program for developing a variety of techniques of

spectroscopy and microscopy to attack this difficulty. These tools are specifically designed

for probing the inhomogeneous distribution of functional thin complex films.

During the second-year effort, several tools had been designed, constructed, and

tested their capabilities in serving our purpose. These techniques include optical retardance

measurement, polarized and time-resolved Fourier-transform infrared (FTIR) absorption

spectroscopy, infrared-visible sum-frequency (IVSFG) vibrational spectroscopy, and Raman

spectroscopy and imaging microscopy.

With the optical retardance measurement set up, anisotropy in the plane of thin films

can be deduced. The in-plane optical anisotropy usually reflects the distribution of

molecular segments that possess polarizable electronic structure and contribute significantly

to the index of refraction. To answer which functional groups are involved in the optical

anisotropic distribution; polarized FTIR with its molecular specificity is employed to provide

more detailed picture. Usually structure and orientation of atomic groups on thin complex

films determined their application properties. An analytical tool with surface sensitivity and

molecular specificity is highly demanded. These requirements can be met with infrared-

visible sum-frequency vibrational spectroscopy. Direct imaging of the infrared absorption

and Raman scattering signals can yield the two-dimensional distribution of specific atomic

groups in thin complex films, is therefore highly useful for probing thin complex films. We

demonstrate these spectroscopic imaging techniques at the micrometer scale to be feasible.

5

Finally, to investigate the molecular dynamics of a field-induced reorientation in a

thin complex liquid crystal film, we are currently undergoing to construct a time-resolved

FTIR spectroscopy to probe the underlying reorientation process at the molecular level. In

the near future, we hope by combining these methods to answer how molecular chirality and

chiral dopants affect the switching dynamics of ferroelectric liquid crystals.

6

Theory

(a) Polar ized infrared absorption and the relations to the order parameters of

molecular or ientational distr ibution

A laser beam propagates through a sample, which has a complex index of refraction

)()()(~ ννν iKnn −= , will experience an intensity loss of

dlINNBc

hdlIKdI nnnn )()( '''''' −==−νν , (IIa.1)

where Nn’ is the population of the n’ level. B is the Einstein B coefficient of the material and

can be related to the square of the transition moment from 'n to ''n

2'''2

32

'''2

'''2

'''2

3

''' |)(|3

)2(]|)(||)(||)(|[

3)2(

nnnnznnynnxnn hhB µπµµµπ

=++= . (IIa.2)

From Eqs. (IIa.1) and (IIa.2), the extinction coefficient K(ν) can be then expressed as

2'''''''''

3

|)(|)(3

)2()( nnnnnn NN

chK µνπν −= . (IIa.3)

Assuming '''nnB to be constant over the spectral line, we then have an integrated absorption

coefficient of

><=∫ 2''' |)(|

3)( nngc

NdK µπνν , (IIa.4)

where KK

Kggg Q∑+= )()0( µµµ .

A 2nd-rank absorbance tensor can be defined as

ΩΩ⋅=Φ ∫ dfEc

NA Kg )(][

3)( 2)(µπ

, (IIa.5)

where Φ is the angle between the incident light polarization direction and the X-axis of the

laboratory coordinates system, f(Ω) denotes the orientational distribution of the dipole

moment derivative in the measurement spot. We can also deduce an anisotropic part from

the absorbance tensor

7

ikikikik ATrAa δ)(31

−= . (IIa.6)

In the molecular frame, the anisotropic absorbance tensor can be expressed in terms of an

1×5 vector

)2,2,2),(21,23( ηςξςξηηηξξςς aaaaaaWmol −= . (IIa.7)

Similarly, in the laboratory frame, the corresponding anisotropic absorbance tensor can be

found to be: )2,2,2),(21,23( yzxzxyyyxxzzlab aaaaaaW −= . (IIa.8)

Fig. IIa.1 Schematic diagram showing the relationship between the molecular frame (ξηζ)

and the laboratory coordinates system (XYZ).

From Eq. (IIa.5) we can obtain simple relationships between the orientational averaged

absorbance tensors from the molecular to the laboratory frames

5,...,2,1,

)()(

=

>><<=ΩΩ=ΩΩ>=< ∫ ∫lmwhere

SWdfSWdfWW mlmol

mmlmol

llab

llab

l . (IIa.9)

With a normal incident geometry on the XZ plane, the integrated absorbance is found to be

>Φ<+−=Φ 2cos232

21

31

)( 21lablab WWAA . (IIa.10)

8

Based on the analysis, the orientational distribution of given functional groups in a

complicated thin film can be deduced from a set of measured polarized Fourier-transform

infrared (FTIR) absorption spectra. The procedure is detailed as follows:

(i) For a given normal mode vibration the integrated absorbance as a function of the

incident infrared polarization direction A(Φ) is first calculated from measured FTIR

spectra;

(ii) Fit the resulted A(Φ) to Eq. (IIa.10) to deduce lablab WandW 21 ;

(iii) From

><+><=−=

><+><==

2221212

2121111

)(21

23

SWSWaaW

SWSWaWmolmol

YXlab

molmolZ

lab

(IIa.11)

and

])()[(21

)(232)(

,2)(

,2

2)(,1

Kg

Kg

mol

Kg

mol

W

W

ηξ

ς

µµ

µ

−=

=

(IIa.12)

the order parameters of molecular distribution, ><><><>< 22211211 ,,, SandSSS , can

then be determined. Note that ><>< 1211 SandS denote the orientational order parameter

and the transverse order parameter (which relates to the dichroic ratio), and

><>< 2221 SandS can reflect the biaxiality of the film.

(b) Raman scatter ing and the relations to the order parameters of molecular

or ientational distr ibution

Considering a molecule which is excited by a laser beam with a polarization along the X-

axis and propagating along the Y-axis of the laboratory frame, the Raman signal along the Z-

axis is proportional to

][2

])()([2

),( 2233

2233

><+><=><+><= XXYXsp

ZYsp

pss cQQ

cI αα

ννπµµ

ννπνν . (IIb.1)

The Raman polarizability tensor for a rod-shape molecule usually possesses a single

dominant component ςςα . A free molecular rotation along the ς -axis leads to the following

orientational averages of the squared polarizability components

9

222242

2442

)]2sin()sin(cos)2cos(sincos2[sin41

]sinsincos[cossin

ςς

ςς

αφφθα

αφφθα

Φ−Φ+ΦΦ>=<

Φ−Φ>=<

YX

YX

.

(IIb.2)

Here Φ is the azimuthal angle between the sample coordinates (xyz) and the laboratory

frame (XYZ). By combining Eqs. (IIb.1) and (IIb.2), we therefore obtain the azimuthal-

dependent Raman scattering intensity

)]2cos(1[)]2cos(2cos(1[),( 00 Φ⋅+=Φ><+= QIII sspss φνν , (IIb.3)

where Q is the azimuthal order parameter. Eq. (IIb.3) will be used to deduce the aligning

ordering of liquid crystal molecules on a variety of polymeric surfaces.

II. Exper imental developments for character izing thin complex films

(a) Character ization of polymer ic aligning sur faces with optical retardation

measurement

The molecular orientation of polymers can be studied using wide-angle x-ray diffraction,

small-angle x-ray diffraction, polarized infrared absorption spectroscopy, and birefringence

measurements. Birefringence measurements are quite robust because they can determine

molecular orientation for both amorphous and crystalline materials. The angle between the

polymer chains and the film in-plane direction govern the anisotropy. The birefringence n∆

of spin-coated films can be related to their molecular orientation >=< )2cos( φQ using

)13(4max

|| +∆

=−=∆ ⊥ Qnnnn (IIIa.1)

where ||n is the index of refraction lateral to the film, ⊥n is the index of refraction vertical to

the film, and maxn∆ is the maximum birefringence at perfect molecular alignment which

yields Q=1.

10

Fig. IIIa.1 Chemical structure (top) of a polyimide (RN-1182, Nissan Chemicals) used in

this study

We prepared a polyimide film with an in-plane anisotropy. The film was subjected to

uniaxial rubbing with a rotating velvet wrapped roller. The resulting in-plane anisotropy was

investigated with an optical retardance measurement. The result is present in Fig. IIIa.2 with

the filled circles. Apparently, the imide and benzene rings, which are believed to be the

species that contribute significantly to the index of refraction of the film. The order

parameter is fairly high with a magnitude of Q=0.72, indicating these rings are aligned very

well by the mechanical rubbing process.

0.010.01 0.030.03 0.040.04

Fig. IIIa.2 In-plane index of refraction change in the polyimide film measured with optical

retardance method. Filled circles (blue): rubbed film, open squares: the same film was

exposed to unpolarized ultraviolet light (λ∼ 250→310 nm) with 147 mJ/cm2, and red crosses:

with exposure 2940 mJ/cm2.

11

The uniaxial alignment can be removed completely by exposing the rubbed film to

unpolarized ultraviolet light (λ∼250→310 nm) with an exposure of 2940 mJ/cm2 (see the

curve of the red crosses in Fig. IIIa.2). Therefore the film has the potential as an aligning

layer of liquid crystal device especially when photoalignment technique is employed. To

reveal the detailed process of the polyimide molecule with ultraviolet light, infrared

absorption and Raman scattering measurements on the alignment layers and liquid crystal

cells were employed and the results are presented as follows.

(b) Character ization of polymer ic aligning films with Four ier transform infrared

absorption spectroscopy

Fourier-transform infrared absorption spectra of RN-1182 polyimide and the film

exposed to UV light with exposures of 0 (blue), 147 mJ/cm2 (red), and 1029 mJ/cm2 (green)

are presented in Fig. IIIb.1. Three major peaks at 1721 cm-1, 1501 cm-1 and 1238 cm-1 can

be attributed to the C=O stretch of the imide ring, C-O-C and C=C stretches of the φ-O-φ

structure, respectively. Two other vibration features with one at 1381 cm-1 and the other at

1154 cm-1 are modified by UV exposure. Based on our ab initio calculation using density

functional theory, we are able to assign these two peaks to the symmetric and antisymmetric

stretches of C-N-C of the imide ring. The peak at 1381 cm-1 was found to upshift to 1397cm-

1 and the other peak at 1173 cm-1 downshift to 1154 cm-1 after UV light exposure.

12

1000 1200 1400 1600 1800

IR Frequency (cm-1 )

0.005

0.015

0.025

0.035

IR A

bsor

banc

e

↓↓

↓

Fig. IIIb.1 Fourier transform infrared absorption spectra of the polyimide film which had

been subjected to varying exposures (blue: 0, red: 147 mJ/cm2 , and green: 1029 mJ/cm2) of

ultraviolet light with λ∼ 250→310 nm.

It has been known that imide ring is photoactive and can be modified by UV absorption. A

simple model can therefore be proposed, which is illustrated by a schematic diagram of Fig.

IIIb.2. The separation of the connected imide rings leads to a rearrangement of the RN-1182

polymer chain and therefore removes the rubbing-induced in-plain anisotropy. A similar

exposure on a spin-coated RN-1182 film with polarized UV light can also produce an in-

plane anisotropy.

13

Fig. IIIb.2 A chemical reaction model proposed to explain the observed infrared absorption

spectra shown in Fig. IIIb.1

To probe more deeply what happen on the rubbed RN-1182 film, we employed infrared-

visible sum-frequency (IVSFG) vibrational spectroscopy to reveal the realigned surface

structure. The result is presented in Fig. IIIb.3, where shows the CH3 stretch of the RN-1182

segments on surface. The peak at 2881 cm-1 is originated from the symmetric stretch of CH3

on Phe-O-Phe segment of RN-1182 molecule. The azimuthal dependence of the peak

intensity along the surface normal clearly exhibits threefold symmetry along the rubbing

direction. Since IVSFG process is high surface specific, the threefold symmetry of the

azimuthal pattern indicates CH3 group on the film surface is not isotropic. We currently are

undertaking similar investigation on UV light exposed RN-1182 films with IVSFG

spectroscopy in order to reveal the detailed surface aligning mechanism by UV photon

absorption. The information is important for understanding the detailed photo alignment

processes of liquid crystal by UV defined polyimide film. The results shall be presented in

our next NSC annual report.

14

2750 2850 2950 3050

Infrared Frequency (cm-1)

0.0

0.3

0.6

SFG

Inte

nsity

(arb

. uni

ts)

0.40.4

rubbed polymer film: ssp SFG Azimuthal pattern

2888 cm-1

Fig. IIIb.3 (Top) An infrared-visible sum-frequency vibrational spectrum from hydrocarbon

segments of a RN-1182 film. (Bottom) Azimuthal dependence of the symmetric CH3 stretch

peak at 2881 cm-1.

(c) Anchor ing energy measurement of liquid crystal molecules on a polymer ic aligning

surface

A film of liquid crystal molecules lying between two aligning surfaces can be

properly modeled by continuum mechanics. The deformation of liquid crystal medium, the

interaction of liquid crystal with an external field and alignment surface can be described

with proper free energies

15

20

2

233

2

022

211

)(21

)(21

)]([)2

()(21

Enf

enAf

nnkp

nnknkf

wherefffF

f

s

b

sfb

×∆−=

⋅−=

×∇×++×∇⋅+⋅∇=

++=

r

rr

rrrrr

εε

π

. (IIIc.1)

Here k11, k22, k33 are the splay, twist and bend elastic constants of the liquid crystal,

respectively, and p0 denotes the pitch of the material induced by a chiral dopant, and

er

represents the easy direction of the aligning surface. By minimizing the total free energy F

with the Euler-Lagrange approach, the following equation can be obtained

θθθ

θθεεφθθθ

θθθθ

233

211

20

222

2

sincos)(

0)(cos)(sin)(])()(

)()(

[21)(

)(

kkf

where

zzzEdzd

ddh

dzd

ddf

dzzdf

+=

=∆+−+

(IIIc.2)

The solution )(zθ is subjected to the boundary conditions at z=0 and z=d

dzedz

zez

Adzdf

Adzdf

==

==

−=

−−=

)](2sin[21

)(

)](2sin[21

)(00

θθθθ

θθθθ . (IIIc.3)

The phase of a light beam after passing through a liquid crystal cell, which is excited with an

external electric field, can be retarded with a magnitude of

]sinsin

[2

2222||

||⊥

⊥

⊥ −+

= nnn

nndRθθλ

π. (IIIc.4)

A set of calculated curves of R as a function of inverse voltage are presented in Fig. IIIb.1. It

shows the anchoring strength A can be deduced from a saturated voltage, with that field

strength all the liquid crystal molecules including that lie on the aligning surfaces are

oriented perpendicularly with the electric field. Therefore with the saturated field strength,

the optical retardance induced by liquid crystal film becomes zero. Experimentally the

16

saturated voltage can be estimated from the measured R-1/V curve by linearly expolating to

the zero retardance point, and the anchoring strength can be calculated from

0.00 0.05 0.10 0.15 0.20 0.25Inverse Voltage (1/V)

0

20

40R

etar

danc

e (D

egre

es)

K11=7 pN Az=5 pN/umK11=7 pN Az=50 pN/umK11=7 pN Az=500 pN/umK11=7 pN Az=20 pN/umK11=7 pN Az=10 pN/umK11=7 pN Az=30 pN/um

HN Cell Cell thickness=10 um

Fig. IIIc.1 Calculated curves of optical retardance as a function of inverse voltage for anti-

parallel aligned nematic liquid crystal cells with varying magnitudes of elastic constant k11

and anchoring strength A.

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.140.0

0.2

0.4

0.6

B A=10 C A=50 D A=100 E A=500 F A=1000 G A=20 H A=10000 I A=80 J A=300 K A=70 L A=40 A antiparallel cell uv0min C antiparallel cell uv1min G antiparallel cell uv3min I antiparallel cell uv7min M antiparallel cell uv20min ### A=150

reta

rdan

ce (r

ad)

1/V

Fig. IIIc.2 Measured (symbols) and calculated (lines) optical retardance of anti-parallel

homogeneous nematic liquid crystal cells aligned by rubbed RN-1182 films with various

anchoring energies.

17

0 1500 3000

Light Exposure (mJ/cm2)

0.5

1.0

1.5

2.0

Anc

horin

g E

nerg

y (1

0-4 J

/m2 )

Fig. IIIc.3 Measured anchoring energy of homogeneous nematic liquid crystal cells

deduced from Fig. IIIc.2. The aligning surfaces were prepared by rubbed RN-1182 films

and then subjected to various unpolarized UV light exposures.

To determine the azimuthal anchoring strength imposed by rubbing action or linearly

polarized ultraviolet light irradiation, 900-twist nematic liquid crystal cell were employed.

Owing to an elastic torque from the twist LC bulk, the liquid crystal molecules on the

surfaces do not orient to yield perfect 090=tφ twist angle. Instead, there exists an angular

deviation 0φ with 00220

0 cossin2

2 φφπφφkAd

pd

t =−− . (IIIc.5)

Equation (IIIc.5) may give us the simplest way to evaluate the azimuthal anchoring strength.

In other words, the anchoring energy can be calculated from the measurement of 0φ as a

zero-field technique.

We also investigate the molecular alignment of nematic liquid crystal molecules on

UV light defined RN-1182 aligning surface with Raman spectroscopy. The aligning surfaces

were exposed with linearly polarized ultraviolet light (LPUV) to induce in-plane anisotropy

[7]. The stretching modes of the C=C and CN groups in a cyano biphenyl liquid crystal

18

molecule can be aligned by LPUV exposed polymer films. This is supported by the

observed strong dependence of the Raman peaks on the rotational angle of the liquid crystal

film along the surface normal as shown by Fig. IIIc.4.

Fig. IIIc.4 Raman spectra from a nematic cyano biphenyl liquid crystal film as a function of

the rotation angle of the film relative to the surface normal. The zero degree denotes the

anisotropic direction of the aligning layer.

The molecular structure of cyano biphenyl liquid crystal molecule is shown in Fig. IIIc.5.

We performed an ab initio density functional calculation to obtain the molecular vibrations

and infrared absorption spectrum. The results are presented in Table IIIc.1

Fig. IIIc.5 Schematic showing the molecular structure of cyano biphenyl (CB) liquid crystal

Four peaks at 1200, 1302, 1616, and 2260 cm-1 can be attributed to from vibrations involved

biphenyl and C-N stretches whose transition moments are mainly along the molecular long

axis. If the long axes of liquid crystal molecules are aligned properly, their Raman scattering

intensities should exhibit significantly azimuthal dependence.

19

Table IIIc.1 Calculated molecular normal-mode frequencies of cyano biphenyl and thecorresponding infrared absorption strengths

Normal Modes Vibration Frequency(cm-1)

IR Absorption Symmetry

1. C1-N stretch 2237 89.8 A1

2. biphenyl C=Cstretch

1605 46.3 A1

3. (a) biphenyl C-C +C-H in-plane wag

(b) C5-C8 stretch +C-H in-plane wag

1446

1261

29.5

6.5

A1

A1

4. C(biphenyl)-H in-plane wag

1095 13.9 A1

1. biphenyl in-planeangular distortion

985 510

4.25.9

A1

6. C-H out-of-planewag

507 544 664 713 742 818

9.96.342.811.845.723.2

B1

0.30.3 0.60.6 0.90.9

1103 cm-1

1199 cm-1

1302 cm-1

1616 cm-1

Fig. IIIc.6 Polar plot of Raman peaks from a nematic cyano biphenyl liquid crystal film as a

function of the rotation angle of the film relative to the surface normal.

20

To probe the ordering, we select the C=C (biphenyl) stretch mode at 1616 cm-1 for

further analysis. The order parameter of the liquid crystal molecules, which are sandwiched

between two polymeric aligning surfaces, can be deduced by using Eq. (IIb.3). The results

are presented in Fig. IIIc.7. Here the polymeric surfaces are rubbed uniaxially and then

assembled anti-parallel. The rubbed surfaces were exposed to unpolarized UV light with

various exposures. It is surprising to see that the unpolarized light exposure enhances the

molecular alignment order instead of destroying it. Light exposure may remove some local

defects from mechanical rubbing, thus increase liquid crystal ordering. Detailed information

can be obtained by employing infrared-visible sum-frequency (IVSFG) vibration

spectroscopy, which is well known for its outstanding surface sensitivity and molecular

specificity. The study with IVSFG has been undertaking.

0 1000 2000 3000Exposure (mJ/cm2)

0.4

0.6

0.8

Ord

er P

aram

eter

Q=<

cosϕ

>

Fig. IIIc.7 Order parameter of the 1616 cm-1 Raman peak from a homogeneous nematic

liquid crystal film at 22 oC (red) and 33 oC (green). The Rubbed polymeric surfaces had

been illuminated with unpolarized UV light to various exposures.

(d) Raman imaging for thin complex films

21

Quantitative optical spectroscopic technique is highly useful for probing various optical

properties of materials. Methods based on the generation of second harmonic (SH) light [1]

and Raman scattering [2] are known to be capable of producing information about the polar

orientation and composition of the sample being studied. In a typical measurement, a sample

is illuminated with a focused laser beam and the generated SH or Raman signal is detected.

The generated signal is then integrated over whole laser beam and pulse duration. The signal

carries information about the material’s properties averaged over the illuminated area. To

probe the local structure and composition of a material, various imaging techniques should

be employed for providing the spatial resolving ability[2,3].

In this section, we shall present a multifunctional optical microscopy with wide field

illumination and multichannel detection. The analytical capability of our instrument was

demonstrated with several application examples in which the polar structure and

composition distribution of materials were resolved.

The Raman imaging spectro-microscope is detailed in Fig. IIId.1. Briefly, laser

excitation is provided by a 523 nm doubled Nd:YLF laser and a wide field excitation is

achieved at the sample. The scattered Raman photons are collected by an infinity-corrected

microscope (Nikon, Eclipse 600) via a Raman edge filter (Omega Optical). For Raman

imaging with high spatial fidelity, the Raman emission is filtered with a compact

electronically controlled 8-cm-1 band pass liquid crystal tunable filter (LCTF)[4]. Raman

images are collected using a 16-bit, TE-cooled (-20 oC), slow scan charge-coupled device

(CCD) detector (Apogee, AP7) having 512 pixels × 512 pixels (25 µm). The CCD has

quantum efficiency more than 80% from 500 nm to 800 nm.

22

Fig. IIId.1 Schematic of a Raman spectroscopy and microscope.

A swing away mirror can be positioned before LCTF to redirect the Raman emission to

a single-stage 0.3m spectrograph (Acton SP300). The Raman spectrum is collected with a

CCD detector (Apogee, SPH4) located at the exit focal plane of the spectrograph.

For SH imaging, a compact mode-locked Nd:YAG laser with a repetition rate of 100

MHz and pulse duration of 18 ps is used [5]. The experimental setup is similar to that

described in Fig. IIId.1 except transmission geometry is employed. A lens with a focal

length of 50 cm focuses the fundamental beam at 1.064 µm to achieve a peak power of 250

MW/cm2. The second harmonic signal at 532 nm was separated from the fundamental with

a series of filters in order to obtain high signal-to-noise ratio.

For probing the material composition, vibrational spectroscopy is better suited than

absorption/luminescence spectroscopy owing to its finger printing capability. By combining

Raman spectroscopy with optical microscopy, a distribution of material composition can be

revealed. This is illustrated in Fig. IIId.2, where Raman spectrum and Raman image from a

silicon integrated circuit chip are presented. The sample had subjected to various oxidation

and metallization processes. The Raman spectrum showed in Fig. IIId.2 (a) exhibits three

peaks located at 520, 610 and 810 cm-1. The 520-cm-1-peak is originated from the lattice

23

vibration of single crystal silicon (SCS) substrate. The other two peaks are mainly from

silicon and oxygen bonding as their intensities are much enhanced compared with the

spectrum taken on a clean SCS (bottom) with a native oxide.

By tuning the LCTF to the corresponding wavelengths of the Raman peaks, the two-

dimensional intensity distributions of the Raman signals can be obtained. In Fig. IIId.2(b),

the color composite (red=520 cm-1, blue=620 cm-1, green=820 cm-1) Raman chemical image

of the sample is presented to reveal the composition distribution of silicon and oxide on the

chip.

.

Fig. IIId.2 (a) Raman spectrum from a silicon integrated circuit chip (top) and a single

crystal silicon substrate (bottom). (b) Color composite Raman chemical image of the 520 cm-

1 (red), 610 cm-1 (blue) and 810 cm-1 (green) peaks from the silicon integrated circuit chip.

In summary, a multifunctional optical microscope has been developed for spatially

resolving the second harmonic and Raman scattering photons. The instrument has been

successfully applied to reveal the polar structure and composition distribution of some

24

optical materials. Detailed analysis of these imaging data demonstrates the technique to be

useful for many other functional materials with high application potential.

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K. Sakamoto, K. Usami, T. Araya, and S. Ushioda, Jpn. J. Appl. Phys. 38, L1435-L1438

(1999).

(e) Time-resolved polar ized Four ier -transform infrared absorption spectroscopy for

liquid crystal alignment and field-induced reor ientation

Vibrational spectroscopy, such as Raman and infrared absorption spectroscopies, are

better suited for probing molecular conformation owing to the finger printing capability. In

liquid crystal research, an important question is that how liquid crystal molecules are aligned

and reoriented during field-induced switching. It is still unclear for the detailed reorientation

dynamics of chain and core parts of liquid crystal. Time-resolved Fourier transform infrared

absorption spectroscopy could yield more useful information.

Recently, Masutani et al. [Appl. Spectro. 46, 560 (1992)] developed a novel

asynchronous time-resolved FTIR spectrophotometer based on a conventional continuous-

scan interferometer. Their new method does not require synchronization between the signal

for time resolving and that for the sampling of the AD converter. In addition, there is no

shortest limit in time for transient phenomena to be measured, therefore time-resolved

25

measurements in the nanosecond or even shorter could be possible if proper gating technique

with infrared laser pulses are available.

In these regards, we modify a commercial FTIR module for making it a time-resolved

FTIR spectrophotometer in order to answer how liquid crystal molecules are aligned and

reoriented during field-induced switching processes. The schematic of our time-resolved

FTIR set up is described in Fig. IIIe.1. The electronic modules in blue color are involved in

the original FTIR modules. The resulting signals at every labeled point are depicted in Fig.

IIIe.2.

Fig. IIIe.1 Schematic showing a time-resolved Fourier transform infrared absorption

spectroscopy with asynchronous continuous scan approach.

Consider a material, which is excited by a periodic excitation )(tτΠ (see Fig. IIIe.2,

A) and the recovery time of the material is shorter than τ. The infrared transmittance

through the excited sample at time t is given then by

)(),()(),( 0 ttTTtT τννν Π⊗∆+= (IIIe.1)

We can also properly express the resulting interferogram as

26

∫ ∑∫

∫ ∫ ∑∫

∫∫

∞+

∞−

∞+

−∞=

∞+

∞−

∞+

∞−

∞+

−∞=

∞+

∞−

+∞

∞−

+∞

∞−

−∆+=

−−∆+=

Π⊗∆+==

ννπντνννπνν

ννπντδνννπνν

ννπννννπνν τ

dxSntTdxStT

dxSdtntttTdxStT

dxSttTdxStTtvtxF

n

n

m

]2cos[)(),(]2cos[)(),(

]2cos[)(]')'()',([]2cos[)(),(

]2cos[)()(),(]2cos[)(),(),2(

0

0

0

(IIIe.2)

By employing a gating technique to sample the interferogram at a fixed delay time

∆tD=gate-excitation, the time resolved interferogram (see Fig. IIIe.2, F) becomes

∫

∫ ∫ ∑∞+

∞−

+∞

∞−

+∞

∞−

+∞

−∞=

∆∆−=

−∆+∆−=

νπνν

ντνπννπνν

τ

τ

dvxStTttG

dntTvxSdvxSTttGtxF

DD

nD

)2cos[)(),()(

),()2cos[)()2cos[)()()[(),(' 0

(IIIe.3)

where T(ν, ∆tD) denotes the time-resolved transmittance at the delay time ∆tD. A smooth

time-resolved interferogram can be obtained by passing the gated signal through a low

pass filter (see Fig. IIIe.2, G)

∫+∞

∞−

∆=∆ νπνντ

dvxStTtxF DD )2cos[)(),(1

),( (IIIe.4)

Fig. IIIe.2 Schematic showing signals at each step of the time-resolved Fourier

transform infrared absorption spectroscopy.

27

We have checked the signals to be properly produced at each step. The data

acquisition in the time-resolved FTIR does not distort the acquired interferograms. This

is confirmed in Fig. IIIe.3, where two the infrared absorption spectra of a polyimide film

for homeotropic liquid crystal alignment measured with a continuous scan (blue color)

and the time-gating setup (red color) are compared. The gating circuit reproduces all the

fine features of the spectrum obtained by a continuous-scan FTIR module.

1000 1200 1400 1600 1800

IR frequency (cm[-1])

-0.05

0.15

IR A

bsor

banc

e

PI for homeotropic alignment

measured by continuous scanmeasured by gating setup

Fig. IIIe.3 Fourier-transform infrared absorption spectra of a polyimide film measured

with a continuous-scan FTIR module (blue) and with gating circuit inserted (red).

IV. Conclusions and future prospect

In this report, we present the studies of complex thin films, such as liquid crystals

and polymeric aligning layers, with optical retardance measurement, infrared absorption

spectroscopy, Raman scattering, and Raman imaging microscopy. The molecular ordering

of liquid crystals and polymeric thin films induced by mechanical rubbing and ultraviolet

light exposure are chosen in view of their technical potential. An important question about

their various applications is how liquid crystal molecules are aligned and reoriented during

28

field-induced switching. It is still unclear for the detailed reorientation process of chain and

core parts of liquid crystal. To understand the underlying dynamics at the molecular level,

we are currently employing Fourier transform infrared absorption spectroscopy to generate

time resolved FTIR spectra of liquid crystal molecules during the field-induced switching

process. By combining these techniques we wish in the near future to answer how molecular

chirality and chiral dopants affect the switching dynamics of ferro- and antiferroelectric

liquid crystals.